1. Field of the Invention
The present invention relates to a compressible printing blanket and a method of manufacturing a compressible printing blanket.
2. Description of the Related Art
A method of manufacturing a laminated printing blanket is disclosed in, for example, U.S. Pat. No. 4,770,928. It is disclosed in this prior art that an intermediate compressible layer is prepared by dispersing microcapsules consisting of a copolymer between, for example, acrylonitrile and vinylidene chloride in an elastomeric material, and that the intermediate compressible layer thus prepared is vulcanized for 1 to 12 hours at a temperature lower than the melting point of the microcapsule, e.g., at about 110° F. (43° C.) to 170° F. (77° C.), so as to fix the microcapsules in place within the structure of the elastomeric material. In this prior art, vulcanization is performed at a low temperature and, thus, used is an accelerator such as dithiocarbamate. It is also taught that after a cloth and a surface rubber layer is further mounted to the vulcanized compressible intermediate layer, vulcanization is applied again at 132° C. to 160° C. so as to obtain a final product. What should be noted is that the vulcanization is carried out twice in this prior art.
Further, U.S. Pat. No. 4,770,928 quoted above teaches in column 3, lines 60 to 66, “As illustrated in the drawing, the voids 28 in the compressible layer 24 have substantially uniform size and substantially uniform distribution and are not interconnected. It has been found that the dimensions of the voids 28 produced in the compressible layer 24 are generally in the same range as the dimensions of the microcapsules used to create voids.” This prior art also teaches in column 5, lines 4 to 5, “The microcapsules used in the method claimed are approximately spherical in shape.” What should be noted is that the voids present in the compressible layer in this prior art are substantially uniform in size and are substantially spherical in shape.
Japanese Patent No. 2670188 is directed to a compressible layer for a printing blanket. This Japanese Patent teaches in column 8, lines 20 to 23, that the intermediate layer has a cellular structure consisting of closed voids, has a uniform thickness, has uniform size voids distributed uniformly, and has the voids not connected to each other.” This prior art also teaches in column 8, lines 41 to 45, “As shown in FIG. 1, these voids 30 are uniform in size and distribution and are not connected to each other. The size of the void 30 formed within the compressible layer 10 has been found to be substantially within the same range as the microsphere used for forming the void.” In conclusion, the voids formed in the compressible layer in this prior art have microsphere shape having a uniform size.
On the other hand, FIG. 2 of Jpn. Pat. KOKAI Publication No. 6-1091 shows that spherical microballoons having a substantially uniform size are present in the compressible layer of a compressible rubber blanket for Example 1.
However, the printing blanket equipped with a compressible layer, which is disclosed in the three prior arts pointed out above, gives rise to the problem that a flat region A in which the compressive stress (kgf/cm2) is not in proportion to the smash thickness (mm) is present over a wide range in the compressive stress curve, as apparent from the graph showing the change in the compressive stress relative to the smash thickness (pushing amount) shown in FIG. 7 included in the accompanying drawings. As a result, a difficulty tends to be brought that the compressive stress is not increased in spite of the increase in the smash thickness, so as to cause the printed surface to become blurred. Also, if the compressibility of the compressible layer is enhanced, the flat region A in the compressive stress curve is further expanded so as to further promote the defect noted above.
Incidentally, FIG. 2 of U.S. Pat. No. 5,364,683 teaches that the density is substantially linearly increased in increasing the smash thickness in the 5056 type printing blanket equipped with a compressible layer manufactured by a salt leaching method.
In the salt leaching method, a mixture of a rubber compound and a soluble salt is vulcanized first, followed by extracting the salt with warm water so as to form a compressible layer. In this method, it is necessary to dispose of the waste water after extraction of the salt so as to make the method troublesome. In addition, the particular method is not desirable in view of the environmental damage. Further, an open void structure is formed in the compressible layer formed by the salt leaching method, giving rise to the problem that the washing solution tends to remain in the compressible layer when the blanket is washed. The washing solution remaining within the compressible layer swells or hardens the compressible layer so as to lower the compressibility of the compressible layer.
Further, Jpn. Pat. Appln. KOKAI Publication No. 10-175381 discloses a printing blanket equipped with a porous compressible layer having closed cells and open cells in a lower section than the surface printing layer. The manufacturing method of the printing blanket is described in paragraphs [0021] to [0022] of this document.
To quote: “an unvulcanized matrix rubber is dissolved into an appropriate solvent, and either a foaming agent or a hollow fine particles that serves to create closed cells CC, and particles used for a leaching method that serve to create open cells OC are blended into the solvent, thus preparing a rubber paste. Thus obtained rubber paste is then applied on a base fabric (, that is, in the example shown in the figure, the upper most fabric of the support layer 11, or the fabric serving as the reinforcement layer 13), and dried and heated, followed by vulcanization.
Here, in the case where the foaming agent is used, the agent is decomposed due to the heat generated in the vulcanization, thus generating gas. In this manner, closed cells CC are created in the matrix rubber. In the case of the hollow fine particles, it is only natural that closed cells CC are created as soon as they are blended thereinto. After that, when the particles used for the leaching method, that are dispersed in thus formed layer are extracted with an appropriate solvent, the imprints of the extracted particles give rise to open cells OC. Thus, the porous compressible layer 12 in which closed cells CC and open cells OC are mixedly present, is formed.”
As described above, open cells OC is made of a number of voids that are coupled with each other. With this structure, when the printing blanket disclosed in the KOKAI Publication is washed, a great amount of washing solution remains in the compressible layer 12, thus causing hardening of the compressible layer 12.